1. Field of the Invention
The present invention relates in general to optical encoders. In particular, the present invention relates a diffraction laser encoder with improved target alignment tolerance.
2. Description of the Related Art
Geometrical optics encoders and diffraction encoders are commonly used for positional and other measurement applications. Both encoders are based on similar principle of operation. Light signal is modulated due to the movement of a grating, and information regarding positional shift and the moving velocity of the grating is obtained via analysis on the received modulated signal. Primary differences between the two are in the method of light modulation and the analytical procedures of the collected light signal.
In a geometrical optics encoder system, grating pitch is typically at the order of about 10 micrometers, which is about 10 times the wavelength of visible laser light sources. Resolution of a geometrical optics encoder system is directly related to the grating pitch, which has a practical limitation. Diffraction phenomenon itself deteriorates signal-to-noise ratio of the system to a level insufficient for practical application. The only way to achieve finer resolution is the refinement of electronics, which is also the major issue in these geometrical optics encoder systems.
In a diffraction encoder system, on the other hand, diffraction grating is typically made at a level of a micrometer, about one-tenth of a typical geometrical optics encoder. In a diffraction encoder system, grating pitch can be reduced for increased resolution as long as operating beams with comparable wavelength is available. Measurement resolution can be improved both electronically and optically (geometrically in a diffraction encoder system. Conventional diffraction encoders, however, are bulky due to their optical component assembly in the system configuration setup. Complex beam path arrangements prevent their application in miniature systems. Miniaturization of these conventional encoder systems sacrifices measurement resolution.